1. Field of the Invention
The present invention is related to a helical antenna with a built-in matching circuit therein, and especially to an antenna which is suitable for communication instruments such as a mobile phone. The helical antenna can make frequency matching in the interior thereof.
2. Description of the Prior Art
Helical structures used as signal receiving and emitting antennae have been well known for a long period. Such helical antennae have been changed a lot in structure and impedance matching in the recent years for they shall simultaneously suit multiple frequencies such as 900 KHZ and 1800 KHZ. For example, in a British patent No. 2,206,243, a lengthy coil and a linear conductor extending into the coil are used to form a dual-frequency helical antenna. And in a Japanese patent No. 1997-320748, a coil provided in an external sleeve can determine its resonance frequency by an extending-in length of an adjusting member extending into the coil from outside of the sleeve.
While the coil element of a helical antenna in the present days can even have sections of different looseness, i.e., a coil can simultaneously suit multiple frequencies. However, no matter how the coil structure and its design are changed, such as for a mobile phone, a problem of matching between the circuit of the antenna and the coil element on a main board is involved. Fundamentally, the circuit of an antenna includes the factors of using capacitors, inductors and grounding etc., different structures of helical antenna surely concern the requisition of using matching elements with different capacity and inductance. However, a main board and an antenna are made by different manufacturers, such requisition of coordination in manufacturing thus is a bothersome matter.